[bin] wrap + unwrap cli support non-TTY (stdin)

[evisong]

This is really a neat tool, thanks!

I'm adding non-TTY (stdin) support to meet my own requirement. Hope it also helps others.

$ npm install -g dynamodb-data-types

$ echo '{"hello":"world"}' | dynamo-dt-attr-wrap
{"hello":{"S":"world"}}

$ echo '{"hello": {"S": "world"}}' | dynamo-dt-attr-unwrap
{"hello":"world"}